Many forms of binary electromagnetic indicators presently exist in the prior art. Two prior art forms of binary electromagnetic indicators are disclosed in U.S. Pat. No. 3,704,462 issued to George E. Pihl on Nov. 28, 1972 and in U.S. Pat. No. 4,115,769 which issued Sept. 19, 1978 to Robert D. Hart and Robert Mazzamauro, both of which patents are incorporated herein by reference.
The Pihl and Hart patents each describe an electromagnetic indicator which is responsive to a fault or malfunction condition, whereby one of a pair of cooperating indicator members, each of which is disc-shaped and mounted along a common axis, rotates relative to the other to change a visual display. Both of the indicator members have a plurality of sectors of like size. The sectors of the stationary indicator member are alternately transparent and opaque, while the sectors of the movable indicator member are totally opaque although alternately distinguishable. In these patents the alternate sectors of the movable indicators are white and dark and the opaque sectors of the stationary indicator member are also dark. For example, the dark sectors of both indicator members may be colored with a black paint. Thus, in one position, the "set" position, the black sectors of the movable indicator member align with black sectors on the stationary indicator member and the white sectors of the movable indicator member align with transparent sectors of the stationary indicator member. In the position to which the movable indicator member moves as a result of a fault or malfunction condition, the opposite alignment will be seen. An alignment of the sectors of the movable indicator member also may be employed such that in the "set" position its black sectors align with the transparent sectors of the stationary indicator member. For purposes of the following description of the invention, particularly in the discussion of the operation of the electromagnetic indicator, it will be assumed that in the "set" position of the indicator members, the display will be totally black and that in the "fault" position of the indicator members, the display will show alternating black and white sectors.
Electromagnetic indicators, such as the electromagnetic indicators disclosed in the Pihl and Hart patents, while generally acceptable, have been found to suffer from certain disadvantages. These disadvantages relate to the ability of the indicator to function properly under typical random high vibrational operating conditions. Since such electromagnetic indicators are frequently used as fault indicators in air craft, the indicators are regularly subjected to severe vibrations and shocks and to strong forces which arise during taking off, landing and maneuvering of the aircraft. Under such adverse operating conditions, previously available electromagnetic indicators frequently slip accidentally from the "set" position to the "fault" position, thereby causing the indicator to display an erroneous reading.